Simple electrical switches typically include an electrically conducting movable element that is coupled to one pole of the switch and which is selectively repositioned to contact another pole of the switch, thereby providing a current path between the two poles, changing the switch from an open state to a closed state. However, this type of mechanical switch requires relatively more space to implement than is often available in many applications and is typically more expensive to manufacture than desired. For example, a keypad used on electrical appliances is typically less than 1 mm thick and can include an array of switches that are each selectively actuated by pressing against an indicated position on the keypad, for example, with the tip of a finger. For such applications, a membrane switch is preferably used, since it can be implemented in a limited depth available for the keypad.
Some membrane switches include electrical contacts or poles attached to a substrate for each switch. A shorting conductive layer overlying the poles of each switch is biased away from the poles and the substrate. When a user presses against a flexible surface supporting the shorting conductive layer, the shorting conductive layer is forced into contact with the poles, changing the switch from an open or non-conducting state to a closed or conducting state.
An example of another type of membrane switch 10, is shown in FIG. 1. Membrane switch 10 includes two substrates 12 and 14 that are planar and generally parallel to each other, but spaced apart. Two switch contacts or switch poles 16 and 18 are formed on substrate 12 and are spaced apart from each other by a gap 20. Poles 16 and 18 can be formed, for example, as the fingers of interdigital electrodes that are plated on substrate 12. A spacer layer 22 is provided around and partially overlying poles 16 and 18. A shorting pad 24 is printed on the undersurface of top flexible layer 14, immediately opposite, but spaced apart from switch poles 16 and 18 by spacer layer 22.
As shown in FIG. 1, prior art membrane switch 10 is in its open or non-conducting state, since shorting pad 24 is not in contact with switch poles 16 and 18. However, if a user applies a force against the upper surface of top flexible layer 14, for example, by pressing with a fingertip, top flexible layer 14 is deflected downwardly by the applied force, causing shorting pad 24 to contact switch poles 16 and 18. The shorting pad then carries an electrical current between switch poles 16 and 18 and membrane switch 10 is in its closed or conducting state.
While membrane switch 10 is very useful in many applications, it has a higher cost than may be desired, because it requires two substrates—substrate 12 on which poles 16 and 18 are applied, and top flexible layer 14 on which shorting pad 24 is applied. Also, since substrate 12 must be separated by spacer layer 22 from top flexible layer 14 to prevent shorting pad 24 from inadvertently contacting the switch poles, this prior art switch must be thicker than is sometimes desired.
To reduce the cost of a membrane switch like that described above, it would be desirable to provide a membrane switch that includes only a single substrate and does not require a top flexible layer as a carrier for a shorting pad. Also, it would be desirable to provide a membrane switch that is thinner in profile (less depth) and responds to an applied force to change the state of the switch more rapidly because there are no moving parts—even without deflecting a substrate to move a shorting pad into contact with electrical switch poles, as in membrane switch 10. It would also be desirable to provide a membrane switch in which the resistance between switch contacts varies as a function of the force applied to the switch. For certain applications, such a switch should enable each of a plurality of switches to be successively activated as a force is applied and swept along a protective sheet overlying the switches. The plurality of switches can be arrayed either linearly or in an arc, i.e., as a rotary membrane switch. It would also be desirable to provide such a switch that should be more durable, since it would have no moving parts.